How It Works
Physical Forms of Fertilizer Urea
Urea is also used in many multi-component solid fertilizer formulations. Urea is highly soluble in water and is, therefore, also very suitable for use in fertilizer solutions (in combination with ammonium nitrate: UAN), e.g., in ‘foliar feed’ fertilizers. For fertilizer use, granules are preferred over prills because of their narrower particle size distribution, which is an advantage for mechanical application.
Urea is usually spread at rates of between 40 and 300 kg/ha but rates vary. Smaller applications incur lower losses due to leaching. During summer, urea is often spread just before or during rain to minimize losses from volatilization (process wherein nitrogen is lost to the atmosphere as ammonia gas). Urea is not compatible with other fertilizers.
Because of the high nitrogen concentration in urea, it is very important to achieve an even spread. The application equipment must be correctly calibrated and properly used. Drilling must not occur on contact with or close to seed, due to the risk of germination damage. Urea dissolves in water for application as a spray or through irrigation systems.
In grain and cotton crops, urea is often applied at the time of the last cultivation before planting. In high rainfall areas and on sandy soils (where nitrogen can be lost through leaching) and where good in-season rainfall is expected, urea can be side- or top-dressed during the growing season. Top-dressing is also popular on pasture and forage crops. In cultivating sugarcane, urea is side-dressed after planting, and applied to each ratoon crop.
In irrigated crops, urea can be applied dry to the soil, or dissolved and applied through the irrigation water. Urea will dissolve in its own weight in water, but it becomes increasingly difficult to dissolve as the concentration increases. Dissolving urea in water is endothermic, causing the temperature of the solution to fall when urea dissolves.
As a practical guide, when preparing urea solutions for fertigation (injection into irrigation lines), dissolve no more than 3g urea per 1 L water.
Advantages of Fertilizer Urea
- Urea can be applied to soil as a solid or solution or to certain crops as a foliar spray.
- Urea usage involves little or no fire or explosion hazard.
- Urea’s high analysis, 46% N, helps reduce handling, storage and transportation costs over other dry N forms.
- Urea manufacture releases few pollutants to the environment.
- Urea, when properly applied, results in crop yield increases equal to other forms of nitrogen.
Incorporate Urea for Best Use
Soil Application and Placement of Urea
If properly applied, urea and fertilizers containing urea are excellent sources of nitrogen for crop production.
After application to the soil, urea undergoes chemical changes and ammonium (NH4 +) ions form. Soil moisture determines how rapidly this conversion takes place.
When a urea particle dissolves, the area around it becomes a zone of high pH and ammonia concentration. This zone can be quite toxic for a few hours. Seed and seedling roots within this zone can be killed by the free ammonia that has formed. Fortunately, this toxic zone becomes neutralized in most soils as the ammonia converts to ammonium. Usually it’s just a few days before plants can effectively use the nitrogen.
Although urea imparts an alkaline reaction when first applied to the soil, the net effect is to produce an acid reaction.
Urea or materials containing urea should, in general, be broadcast and immediately incorporated into the soil. Urea-based fertilizer applied in a band should be separated from the seed by at least two inches of soil. Under no circumstances should urea or urea-based fertilizer be seed-placed with corn.
Depending on soil and climate good crop production could requires an application of more than 20 lb. of nitrogen per acre. Farmers can avoid damage from urea by broadcasting most of the urea nitrogen fertilizer ahead of seeding.
Spreading of Urea
Urea can be bulk-spread, either alone or blended with most other fertilizers. It is recommended that the spreading width not exceed 50 feet when combined with other fertilizer materials.
Urea often has a lower density than other fertilizers with which it is blended. This lack of “weight” produces a shorter “distance-of-throw” when the fertilizer is applied with spinner-type equipment. In extreme cases this will result in uneven crop growth and “wavy” or “streaky” fields.
Blending Urea with Other Fertilizers
Urea and fertilizers containing urea can be blended quite readily with monoammonium phosphate (11-52-0) or diammonium phosphate (18-46-0).
Urea should not be blended with superphosphates unless applied shortly after mixing. Urea will react with superphosphates, releasing water molecules and resulting in a damp material which is difficult to store and apply.
Uniformity of particle size is important with dry solid urea, whether applied directly or in blended formulations.
The liquid mix of urea and ammonium nitrate (UAN 28% N) has been on the market for a long time. The characteristics of this solution, however, are not the same as when urea alone is dissolved in water. A solution of 50% urea by weight results in 23-0-0 and has a salting-out temperature of 60 degrees F. In order to store and handle liquid urea during cooler temperatures, the nitrogen concentration must be lowered to reduce salting problems. There are several possible formulations that can be used for this, such as adding small amounts of ammonium nitrate, ammonium sulfate, or anhydrous ammonia.
Research, particularly on liquid urea, is very limited. Generally, where dry urea functions successfully, the fluid urea should perform equally well and may have the advantage of better uniformity over some dry urea sources.
Biuret in Urea
The most common impurity of synthetic urea is biuret, which impairs plant growth.
Biuret in urea can cause agronomic problems if placed near the seed, or even if added preplant in bands where seeds will later be planted.
It is advisable to keep biuret content low by keeping high temperatures to a minimum. Biuret content is typically around 0.3%.
High heat is normal during the manufacture of urea. If heat exceeds 200 degrees F there is a slight conversion of urea to biuret, but this takes place only during the manufacturing process. No such conversion happens in storage or in the soil.
Biuret converts to ammonia, but conversion is much slower than for urea. Since biuret remains in the soil for several weeks, the potential for seed damage continues beyond the brief period of conversion of urea to ammonia. The major damage of biuret is to germinating seeds. There is little damage through plant absorption, although some citrus crops have been affected.
Application of Urea to Growing Crops
Urea can be applied to sod crops, winter wheat, or other small grains. This application, however, should be made during cool seasons. During warm periods (60 degrees F or above), urea in contact with vegetative material will tend to give off ammonia.
If urea must be applied on grass pastures in the summer, apply when there is a high probability of rainfall.
Foliar Application of Urea
Urea can also be applied as a foliar spray on some crops, such as potatoes, wheat, vegetables, and soybeans. Urea is highly water soluble. At normal atmospheric temperatures, approximately 1 lb. of urea can be dissolved in 1 lb. of water.
Research data indicate that urea should contain no more than 0.25% biuret for use in foliar sprays. For many crops the quantity of nitrogen applied at one time should not exceed 20 lb. of nitrogen per acre.
SLOW RELEASE OF UREA
Urea fertilizer can be coated with certain materials, such as sulfur, to reduce the rate at which the nitrogen becomes available to plants. Under certain conditions these slow-release materials result in more efficient use by growing plants. Urea in a slow-release form is popular for use on golf courses, parks, and other special lawn situations.
UREA DO’S AND DON’TS
- Store separately from ammonium nitrate.
- Do not use small, fast-moving augers to move the urea.
- Do not exceed a spreading width of 50 feet when urea is applied.
- Do not place in direct contact with corn seed.
- Keep rates of nitrogen applied together with small grain in drill to 10 1b on dry soils, 20 lb when soil is moist.
- Apply urea on sod crops when atmospheric temperature is below 60 degrees F.
- When urea is broadcast on soils of high pH (above 7.5), the material should be incorporated into the soil as soon as possible.